The question is will it be a straight line change or hyperbolic?
Considering the 2 billion more people on the planet by 2050 and the non-straight
line increase in Chinese and other countries' development, then climate
temperatures should be more than the straight line if nothing momentous is done
to contain it.

So far, we've raised the average temperature of the planet
just under 0.8 degrees Celsius, and that has caused far more
damage than most scientists expected.
(A third of summer sea ice in the
Arctic is gone, the oceans are 30 percent
more acidic )
Many scientists have come to think that two degrees
is far too lenient a target. "Any number much above one
degree involves a gamble," writes Kerry Emanuel of MIT, a leading
authority on hurricanes, "and the odds become less and less favorable as the
temperature goes up." "two degrees is simply too much."
NASA scientist James Hansen, the planet's most prominent climatologist,
is even blunter: "The target that has been talked about in international
negotiations for two degrees of warming is actually a prescription for
long-term disaster." Some small island nations
will flat-out disappear." "two degrees would represent a "suicide pact" for
drought-stricken Africa. "

1999-2012 flattening temperatures

OVER the past few years one of the biggest questions in
climate science has been why, since the turn of the century, average
surface-air temperatures on Earth have not risen, even though the concentration in the atmosphere of heat-trapping
carbon dioxide has continued to go up. This “pause” in global warming has been
seized on by those sceptical that humanity needs to act to curb greenhouse-gas
emissions or even (in the case of some extreme sceptics) who think that man-made
global warming itself is a fantasy. People with a grasp of the law of
conservation of energy are, however, sceptical in their turn of these positions
and doubt that the pause is such good news. They would rather understand where
the missing heat has gone, and why—and thus whether the pause can be expected to
continue.

The most likely explanation is that it is hiding in the oceans, which store nine times as much of the sun’s
heat as do the atmosphere and land combined. But until this week, descriptions of how the sea might do this have
largely come from computer models. Now, thanks to a study published in
Science by Chen Xianyao of the Ocean University of
China, Qingdao, and Ka-Kit Tung of the University of Washington, Seattle, there
are data.

Dr Chen and Dr Tung have shown where exactly in the sea the
missing heat is lurking. As the chart below shows, over the past decade and a
bit the ocean depths have been warming faster
than the surface. This period corresponds perfectly
with the pause, and contrasts with the last two decades of the 20th century,
when the surface was warming faster than the deep. The authors calculate that,
between 1999 and 2012, 69 zettajoules of heat (that is, 69 x 1021
joules—a huge amount of energy) have been sequestered in the oceans between
300 metres and 1,500 metres down. If it had not been so sequestered, they
think, there would have been no pause in warming at the surface.

Hidden depths

The two researchers draw this conclusion from observations
collected by 3,000 floats launched by Argo, an international scientific
collaboration. These measure the temperature and salinity of the top 2,000
metres of the world’s oceans. In general, their readings match the models’
predictions. But one of the specifics is weird.

Most workers in the field have assumed the Pacific Ocean would
be the biggest heat sink, since it is the largest body of water. A study
published in Nature in 2013 by Yu Kosaka and Shang-Ping
Xie of the Scripps Institution of Oceanography, in San Diego, argued that
cooling in the eastern Pacific explained most of the difference between actual
temperatures and models of the climate that predict continuous warming. Dr
Chen’s and Dr Tung’s research, though, suggests it is the
Atlantic (see middle chart) and the Southern Ocean that are doing the sequestering. The
Pacific (right-hand chart), and also the
Indian Ocean, contribute nothing this
way—for surface and deepwater temperatures in both have risen in parallel since
1999.

This has an intriguing implication. Because the Pacific has
previously been thought of as the world’s main heat sink, fluctuations affecting
it are considered among the most important influences upon the climate. During
episodes called El Niño, for example, warm water from its west sloshes eastward
over the cooler surface layer there, warming the atmosphere. Kevin Trenberth of
America’s National Centre for Atmospheric Research has suggested that a strong
Niño could produce a jump in surface-air temperatures and herald the end of the
pause. Earlier this summer, a strong Niño was indeed forecast, though the
chances of this happening seem to have receded recently.

But if Dr Chen and Dr Tung are right, then the fluctuations in
the Atlantic may be more important. In this ocean, saltier tropical water tends
to move towards the poles (surface water at the tropics is especially saline
because of greater evaporation). As it travels it cools and sinks, carrying its
heat into the depths—but not before melting polar ice, which makes the surface
water less dense, fresh water being lighter than brine. This fresher water has
the effect of slowing the poleward movement of tropical water, moderating heat
sequestration. It is not clear precisely how this mechanism is changing so as to
send heat farther into the depths. But changing it presumably is.

Understanding that variation is the next task. The process of
sequestration must reverse itself at some point, since otherwise the ocean
depths would end up hotter than the surface—an unsustainable outcome. And when
it does, global warming will resume.